1. Field of the Invention
The present invention relates to vacuum electron devices (VEDs). More particularly, the present invention relates to input circuits for high power RF amplifiers which employ VEDs such as Klystrodes, Inductive Output Tubes (IOTs), and the like in the television broadcast service.
2. The Background Art
Vacuum tube amplifiers generally include an input circuit having three major components: the enclosure, the input resonator, and the socket. The enclosure houses the socket and the input resonator to which high voltage connections are made. Not only does the enclosure envelope the circuit, but its function is also to contain radio frequency (RF) energy within the RF compartment.
IOTs have limited life times and must be replaced from time to time. Existing IOT-based amplifier designs generally require complete removal of the amplifier input circuit from the transmitter in order to replace the VED. This process can be cumbersome and inconvenient. During tube replacement, electrical contact fingers in the socket may be easily damaged due to incorrect alignment. With damage to the contact fingers, RF energy may leak from the amplifier. RF leakage can also generate a substantial amount of heat or arcing which may damage wiring and components. In addition, misalignment may also cause RF leakage from the amplifier enclosure due to improper seating on an electro magnetic interference (EMI) gasket.
Even if the input circuit is properly seated, the high voltage leads can couple an undesirable percentage of the input RF into the transmitter's instrumentation. Due to spatial constraints, it is difficult to isolate the RF signals within the enclosure by loading it with ferrites (filter components, chokes and bobbins). Consequently, end-users currently place such RF isolation components in the transmitter output circuit. Despite the ability to combine RF components and high voltage components under the same cover, the spatial constraint limits the ability to improve the product. Aside from RF isolation, high voltage standoff issues make it difficult to incorporate a quick and easily accessible connection box.
FIG. 1 is an external perspective view of a conventional input circuit and enclosure of an amplifier employing a VED in accordance with the prior art. An enclosure cover 10 houses a radio frequency (RF) connection and high voltage connections to a VED (not shown). An air distribution system comprising a tree 12 and branches 14 access the VED enclosure through a separate entry 16 from cover 10.
FIG. 2 is a cross-sectional drawing of an input resonator and socket for a VED in accordance with the prior art. The input resonator comprises a parallel LC circuit. The inductance is provided by a shorting pin (not shown) located between the cathode 21 and grid 24 lines. The capacitance is generated by a cathode and grid structure (not shown) located in the VED. The input resonator is capacitively tuned such that the structure's parallel circuit resonant frequency matches the operational carrier frequency the VED is operated at. The cathode 21 and grid 24 lines also serve as socket collets which affix to their corresponding surfaces on the VED (not shown). The collets transfer the input RF energy to the input section of the VED. In addition, the cathode line delivers the DC beam voltage to the VED's cathode. The grid line distributes the bias voltage to the VED's grid. The socket is also comprised of a heater collet 25 and a vac-ion 31 contact. The heater collet delivers a DC voltage to the VED to provide power needed to operate the VED's cathode (not shown) at an elevated temperature. The vac-ion contact provides a DC voltage required to operate an appendage vacuum pump (not shown) located on the VED.
In operation, an alternating RF voltage is applied between the cathode 21 and grid 24 lines. The input RF voltage propagates to the input section of the VED (not shown) generating a RF voltage between the VED's grid and cathode (not shown). The VED's cathode emits electrons resulting in a bunched (density modulated) electron beam. An anode structure (not shown) operating at a high DC beam voltage accelerates the bunched beam through the anode's aperture.
The heater collet 25 is retained to cathode lines 21 and 22 through C-Clips 26 as heater collet 25 heats up cathode lines 21 and 22. Mounting screws 27 retain heater collet 25 against a high voltage insulator 28. When heater collet 25 needs to be removed for maintenance, mounting screws 27 along with C-clips 26 must be disassembled. Therefore, when a user needs to replace a component of the RF socket that houses the heater line, the entire RF socket needs to be completely removed. Such components can easily be damaged during assembly or installation of the RF socket.
Accordingly, a need exists for an improved input circuit for an RF amplifier providing a high power output which provides a good seat alignment for the VED with an EMI gasket to prevent RF leakage, an easy assembly and disassembly mechanism, a proper cooling system with RF isolation, and an easy socket interface.